Studies involve collaboration within the Laboratory on the characterization of neoplastic cells and the study of tumor histogenesis. The aim of the present investigation is to define by high resolution and immunoelectron microscopy changes in natural killer cells and targeted tumor cells which correlate with natural killer cell activity. Large granular lymphocytes (LGL) have been identified with natural killer cell activity. OX-8, a monoclonal antibody which reacts with a cell surfact antigen of LGLs and cytotoxic/suppressor T-lymphocytes, has been used in the immunoelectron microscopy of lymph nodes from nude rats which lack the T-cells. A postembedding staining procedure gave moderate preservation of lymphoid cell structure. Cells whose membranes were specifically stained by the antibody displayed characteristics consistent with their being LGLs (kidney-shaped nucleus, cytoplasmic granules). Nonlymphoid cells were, however, poorly preserved. Current investigations involve the establishment of a staining protocol which gives acceptable ultrastructural preservation of all cell types as well as specific antibody staining.